The present invention relates to radiation therapy equipment and in particular to a method and apparatus of using intensity modulated radiation therapy equipment for the treatment of targets subject to respiratory motion such as tumors related to lung cancer and liver cancer.
Intensity modulated radiation therapy (IMRT) systems treat a tumor with converging “beamlets” of radiation (being separately controllable portions of a radiation beam) of different intensities. The overlapping pattern of the beamlets allows the delivered radiation dose to closely conform to complex tumor shapes while minimizing radiation to adjacent tissue.
In first generation IMRT, a two-dimensional multileaf collimator, of a type normally used to outline the radiation beam, is used to control beamlets of a two-dimensional radiation beam according to a speed of movement of each collimator leaf. Radiation is delivered to the patient from a limited number of angles and the patient is stationary. Equipment suitable for conventional IMRT is described in U.S. Pat. No. 4,868,843, hereby incorporated by reference.
In second generation IMRT, such as is taught in U.S. Pat. Nos. 5,317,616; 5,548,627; 5,673,300; 6,438,202; 6,560,311 assigned to the assignee of the present invention and hereby incorporated by reference, a one-dimensional multileaf collimator modulates beamlets of a fan beam by controlling an opening dwell time of each collimator leaf. Radiation is delivered in a plane over 360 degrees and the patient is translated helically or after each rotation to treat a tissue volume.
The pattern and intensity of the different beamlets is determined by a treatment plan typically based on computed tomography (CT) images of the patient on which the tumorous tissue is identified. Computation of beam intensities based on the tumor position and shape uses complex optimization algorithms and is normally done well in advance of the treatment.
The treatment of targets having respiratory motion such as non-small cell lung cancer presents a problem for treatment planning for IMRT equipment because the tumor moves with movement of the lungs over the course of the treatment. Currently, one of three techniques is used to address this problem: gating, breath-hold, and chasing. In the gating technique, patient breathing is tracked and treatment is suspended for phases of the breathing cycle when the tumor is displaced excessively from the location assumed by the treatment plan. Such an approach increases the length of time required for the radiation therapy and, by implicitly allowing some tumor motion during treatment, cannot achieve the highest dose conformity possible with IMRT.
In the breath-hold technique, the patient holds his or her breath on cue or by operation of a valve system. Particularly for patients with lung disease, such breath-hold techniques can be difficult. Because treatment is performed only during breath-holds, the length of time of the treatment over which the treatment is performed is increased.
In chasing techniques, the patient is allowed to breathe regularly following a predetermined pattern and the tumor trajectory is monitored using either a respiratory signal or an implanted marker. The radiation beam is then steered to follow a computed average tumor trajectory by superimposing the tumor movement on motion of movable leaves collimating the radiation beam.